β-Type zeolites are high-silica zeolites which were disclosed first by patent document 1, and are in extensive use as catalysts, adsorbents, etc.
Before being utilized, zeolites are frequently modified chemically with metals for the purpose of imparting a specific function, e.g., catalytic activity or adsorption selectivity. Methods in general use include: a method in which metal cations are loaded into a zeolite by means of ion exchange in a liquid phase while utilizing the ability of the zeolite to undergo ion exchange; and a method in which a zeolite is impregnated with a solution containing a salt of a metal to thereby load the metal into the zeolite.
In chemical modification of a zeolite, it is generally desirable, from the standpoint of enabling the desired function to be exhibited to a highest degree, that the metal loaded should be present in the state of having been highly dispersed in the zeolite base and have aggregated as less as possible.
However, in the loading based on ion exchange described above, the amount of the metal which can undergo the exchange depends on the ion-exchange ability of the zeolite, and an attempt to load more than that is apt to result in metal aggregation. On the other hand, the loading through impregnation has a problem that metal aggregation becomes more apt to occur as the amount of the metal to be loaded is increased, although it is easy to control the amount of the metal to be loaded.
Some metals can be introduced into the zeolite framework like silicon and aluminum by adding the metals as a starting material for hydrothermal synthesis of a zeolite. According to this method, atoms of the metals being introduced are separately incorporated into the silicon network including oxygen atoms and, hence, the metals come into an exceedingly highly dispersed state. Known metals which can be introduced include B, Cr, V, Ge, Ga, Fe, Sn, Zn, and the like. A relatively large number of attempts to introduce iron, among these metals, have been made so far.
With respect to β-type iron silicates obtained by introducing iron into the framework of a β-type zeolite, the following prior-art techniques have been disclosed.
For example, patent document 2 discloses an adsorbent including a β-type iron silicate which has both aluminum and iron in the framework thereof. Patent document 3 discloses an adsorbent for automobile exhaust removal which includes a β-type iron silicate specified with, for example, the full width at half maximum of an X-ray diffraction peak. Moreover, patent document 4 discloses an aluminosilicate zeolite which contains both framework iron and iron cations present on the ion-exchange sites.
Furthermore, β-type iron silicates are disclosed also in, for example, non-patent document 1 and non-patent document 2. In addition, non-patent document 3 discloses a β-type iron silicate synthesized from starting materials into which fluorine had been added. It is generally known that by adding fluorine to starting materials in zeolite synthesis, a zeolite having fewer lattice defects and better crystallinity is obtained as compared with the case where fluorine is not added. For example, such techniques for a β-type zeolite are disclosed in non-patent document 4.
However, the β-type iron silicates disclosed in those documents are β-type iron silicates in which the amount of the introduced iron is relatively small or which contain a large amount of iron introduced but contain a large amount of aluminum that coexists within the crystals, or are β-type iron silicates which are presumed, from the disclosed crystalline form, to have undergone unsatisfactory crystal growth. The reasons for this are that in the hydrothermal synthesis of a general iron silicate, as the content of aluminum in the starting materials decreases, the range of starting-material compositions capable of crystal formation becomes narrower and the reaction comes to require an exceedingly prolonged period, as compared with the case of ordinary aluminosilicates. Namely, in the method of introducing iron through hydrothermal synthesis, it is difficult to introduce a large amount of iron while minimizing coexistence of aluminum, although iron can be introduced in a highly dispersed state.
Meanwhile, industrial use of fluorine in zeolite synthesis is difficult from the standpoints of corrosion of the equipment, etc. Furthermore, use of fluorine is undesirable because there is a possibility that the fluorine remaining in the zeolite yielded might adversely affect the performance.
However, in the field of applications such as catalysts or adsorbents, there has been no known β-type iron silicate having high heat resistance and high crystallinity in which iron that functions as solid-acid sites and is capable of functioning as catalytically active sites, adsorption sites, or the like is contained in a highly dispersed state in the crystals in a far larger amount than conventionally known amounts and which has been synthesized without using fluorine, which is difficult to use industrially.
The β-type iron silicates disclosed in those documents are β-type iron silicates in which the amount of the introduced iron is extremely small and, hence, the iron is not always sufficient for use as active sites in catalysts, etc., or are β-type iron silicates which have insufficient crystallinity although aluminum and iron have been introduced in sufficient amounts. The reason for this is as follows. In the hydrothermal synthesis of an aluminum-containing iron silicate, there generally is the following tendency. As the iron content of the starting materials decreases, the properties of the product become more akin to the properties of ordinary aluminosilicates, so that crystallization becomes easier and, simultaneously therewith, crystals having better crystallinity are obtained. In contrast, as the iron content of the starting materials is increased, the region for crystal formation decreases rapidly and crystals having reduced crystallinity are obtained. Such tendency is thought to be attributable to the instability of the iron as compared with the aluminum in the alkaline starting-material mixture and to the enhanced crystal distortion due to the presence of iron, which has a larger ionic radius than aluminum, in the zeolite framework.
Nitrogen oxide removal catalysts constituted of a crystalline silicate which has a β-type structure including iron and has an SiO2/Fe2O3 ratio of 20-300 and in which the proportion of isolated iron ions in the iron contained is 80% or more (patent document 5) have been reported. In these catalysts, however, the crystalline silicate has been made to have an increased crystal grain diameter and enhanced crystallinity by using a fluorine source.
It has also been reported that crystalline silicates which have a β-type framework structure including iron and which have an SiO2/Fe2O3 molar ratio of 20-300 and a log(SiO2/Al2O3) value of 2 or more (molar ratio) have the excellent ability to decompose nitrogen oxides at low temperatures and excellent hydrothermal durability (patent document 6). However, these crystalline silicates also have a grain diameter as large as 5 μm or more.
There has so far been no known β-type iron silicate having high heat resistance and high crystallinity which is synthesized without using fluorine, which is difficult to use industrially, and contains aluminum serving as solid-acid sites and iron capable of functioning as catalytically active sites or adsorption sites or the like, both the aluminum and the iron being contained in a highly dispersed state in the crystals, and which has a grain diameter range that enables the silicate to be expected to show high dynamic performance when used as a catalyst.
Crystalline silicates in which an element of another kind has been substituted in the framework structure are expected to show properties different from those of ordinary aluminosilicate zeolites, and use of the crystalline silicates in catalytic reactions is being investigated. For example, disclosed techniques include a xylene isomerization catalyst which employs an iron silicate loaded with platinum (patent document 7), a catalyst for the selective methylation of naphthalenic compounds which employs an iron silicate (patent document 8), a process for polyalkylene glycol production using an iron silicate as a ring-opening polymerization catalyst for cyclic ethers (patent document 9), etc.
Meanwhile, investigations are being made also on techniques for removing nitrogen oxides using an iron silicate. For example, the following techniques have been reported: a catalyst for cleaning discharge gases containing nitrogen oxides, the catalyst including a ZSM-5 iron silicate into which a coprecipitated composite oxide of copper and gallium has been dispersedly loaded (patent document 10); a method for removing nitrogen oxides in which a ZSM-5 iron silicate that has been ion-exchanged with an alkali metal is brought into contact with a discharge gas containing nitrogen oxides, in an atmosphere containing excess oxygen in the presence of a hydrocarbon or an oxygen-containing compound (patent document 11); a method for removing nitrogen oxides in which a discharged combustion gas that contains nitrogen oxides, oxygen gas, and optionally sulfurous acid gas is catalytically reacted in the presence of an iron silicate catalyst and a hydrocarbon as a reducing agent (patent document 12); and a discharge gas removal catalyst for mainly removing nitrogen oxides, the catalyst having been obtained by loading at least one of platinum, palladium, rhodium, and cobalt into an iron silicate (patent document 13). Incidentally, the iron silicates described in patent documents 12 and 13 each are considered to have a ZSM-5 framework structure because the iron silicates were obtained using a tetrapropylammonium salt in the synthesis thereof.
With respect to catalysts for removing nitrous oxide, the following techniques have, for example, been disclosed: a process for producing a catalyst which includes a β-type iron silicate loaded with copper, cobalt, or the like and which is for use in direct decomposition of nitrous oxide (patent document 14); a method in which an iron silicate having a β-structure is used to directly decompose nitrous oxide; and a method in which carbon monoxide is used as a reducing agent to catalytically and non-selectively reduce nitrous oxide (non-patent document 3).
Meanwhile, with respect to catalysts for removing nitrogen oxides contained in discharge gases, a method is known in which an aluminosilicate zeolite catalyst loaded with iron or copper is used to conduct selective catalytic reduction (generally called SCR) with ammonia for the purpose of removing the nitrogen oxides contained in an exhaust gas containing oxygen in excess, which is represented by lean-burn exhaust gas and diesel exhaust gas (patent document 15).
Nitrogen oxide removal catalysts constituted of a crystalline silicate which has a β-type structure including iron and has an SiO2/Fe2O3 ratio of 20-300 and in which the proportion of isolated iron ions in the iron contained is 80% or more (patent document 5) have been reported. However, when used in a method for reducing nitrogen oxides (NOX) using ammonia as a reducing agent, these catalysts were insufficient in the ability to decompose nitrogen oxides at low temperatures and in hydrothermal durability.
Furthermore, it has been reported that crystalline silicates which have a β-type framework structure including iron and which have an SiO2/Fe2O3 molar ratio of 20-300 and a log(SiO2/Al2O3) value of 2 or more (molar ratio) have the excellent ability to decompose nitrogen oxides at low temperatures and excellent hydrothermal durability (patent document 6). However, these crystalline silicates have a crystal grain diameter as large as 5 μm or more, and still have a problem concerning handling, such as coating, molding, etc.